1. Field of the Invention
The current invention relates to conical microstructures of cubic boron nitride and to arrays of such conical microstructures. The invention also relates to fabrication cubic boron nitride cone microstructures.
2. Background Information
Alike diamond, cubic boron nitride (cBN) is a material with extreme properties. It has the second highest hardness and second highest thermal conductivity next to diamond, wide band gap (˜6.4 eV), high optical transparency up to the deep ultraviolet spectral region, extreme chemical inertness, and high dielectric strength. However boron nitride (BN) surpasses diamond in graphitization and oxidation temperatures and chemical inertness. cBN is inert to molten ferrous materials, but diamond is dissolved in these materials. The combination of these extreme mechanical, thermal and chemical properties makes cBN ideal material for cutting tools and tribological coatings involving steels and ferrous materials. Cubic BN is also a better material for production of high-power electronic devices operating at high temperature, high switching frequency and in harsh irradiative environment because of its wide band gap properties, expected high carrier mobilities and capacity of doping for either p-type or n-type conductivities [O. Mishima, J. Tanaka, S. Yamaoka, and O. Fukunaga, Science vol. 238, pps. 181-183, “High-temperature cubic boron nitride p-n junction diode made at high pressure”, 1987].
Cubic BN powders synthesized by high-pressure high-temperature (HPHT) method have been commercially used in cutting tools and wear parts with metal cementing technique. However, the cBN powders usually have grain sizes ranging from submicron to millimeters [N. V. Novikov, Diamond Relat. Mater., vol. 8, pps. 1427-1432, “New trends in high-pressure synthesis of diamond”, 1999], which constrains its application, especially in electronics. The practical application of cBN in film forms has also been hampered because the incompatibility of cBN with many types of substrates primarily in their physical properties and the process of synthesis, which leads to the poor cBN film properties including sensitivity to humidity, excessive stress, and delamination of film thicker than 200 nm. Recent novel approaches in synthesis, lowering the particle energy, introduction of fluorine chemistry, growth on diamond based substrates and use of gradient layers have provided thicker and well adherent films. Due to the compatibility in structural and physical properties of cBN and diamond, the introduction of diamond interlayer eases the cBN synthesis and even enables heteroepitaxial growth of cBN films. For example, using an electron cyclotron resonance (ECR) microwave plasma (MP) chemical vapor deposition (CVD) and a gas mixture of He—Ar—N2—BF3—H2 allows us to deposit several μm thick cBN films over large areas on diamond substrates (˜3 inches in diameter). (Zhang W J, Bello I, Lifshitz Y, Chan K M, Wu Y, Chan C Y, Meng X M, Lee S T, Appl. Phys. Lett., vol. 85, pps. 1344-1346, “Thick and adherent cubic boron nitride films grown on diamond interlayers by fluorine-assisted chemical vapor deposition”, 2004). These cBN films grow directly and epitaxially on polycrystalline diamond substrates. The soft sp2-BN incubation layer usually needed for cBN nucleation is absent. Thus the adhesion and crystal quality of the cBN films are much improved, which foresees exciting applications of cBN/diamond composite films (cBND) in protective, tribological and electronic applications.
Micro- and nano-conical structures have increasingly become of interest due to their special electronic and mechanical features. Diamond and silicon cones with high aspect ratios have been developed for the use in field emitting devices because of geometrical field emission enhancement and their time stability. Examples can be found in U.S. Pat. Nos. 6,762,543 and 5,627,427 and in the work of Zhang et al [Zhang W J, Wu Y, Chan C Y, Wong W K, Meng X M, Bello I, Lifshitz, Lee S T, Diam. Relat. Mater., vol. 13, pps. 1037-1043, “Structuring single- and nano-crystalline diamond cones”, 2004]. Single crystalline diamond cones with small tip radius, high aspect ratio and defined crystal orientation as disclosed in U.S. Pat. No. 6,902,716 have the advantage in improving the resolution of AFM probes as well in other SPM.
Because of the superior properties surpassing diamond, cBN structures are exceptionally durable materials that could have even better electronic qualities than carbon counterparts. Due to the second highest hardness, elastic modulus, and thermal conductivity next to diamond, and doping capacity for both n- and p-type conductivity, a single cBN cone is also of great interest in many experimental methods of analysis and testing, for example, scanning probe microscopy (SPM), nanoindentation and other nanoprobe techniques. Furthermore, cBN cone with a large surface area benefits to improve the efficiency and sensitivity in the electrode and sensor applications. Thus, cBN cone could open new frontier in electroanalysis and stable sensor applications. Such cones can be used as a pressure sensor because of the stability of its crystal structure in extreme conditions. Cathodes are used in a number of electronic devices such as displays, power amplifiers and vacuum microelectronics. Conventional cathodes are relatively low current devices which require either high extraction voltages or elevated temperatures for operation. Accordingly, it would be desirable to provide a cold cathode which would function at lower temperatures and voltages than existing cathodes. Since cBN has negative electron affinity, there is a potential application of cBN in field emitting devices. However, availability of suitable cBN films and then their high switch-on electric field have hampered using them in these applications. The formation of high-density sharp cones of cBN would certainly decrease the switch-on electric field and promote its field emission applications.
Diamond pyramids are easily made by fabricating an array of pyramidal pits in a silicon substrate by anisotropic etching and filling the pits by CVD diamond and then removing the silicon substrate. However, Cubic BN nanostructures such as nanocones have not been possible hitherto because of the significant difficulties in preparing fundamental structures of cBN films with satisfactory quality and thickness over large areas.